USB powered devices have become ubiquitous because of the popularity of computers. However, the voltage supplied by a USB connection has a characteristic that limits the use of USB powered devices. Specifically, the output voltage of the USB port will decrease when the output current from the USB port is larger than some value, for example, 500 mA. So there is a need to limit the current supplied by the USB port in order to allow proper use of USB powered devices.
FIG. 1 illustrates one prior art peak current limiting method for current limit. The current (known also as input current in FIG. 1) flowing through the switch Sin is sensed and compared with a threshold VCLM. The switch Sin is turned off when the sensed current is larger than the threshold VCLM, thus limiting the peak value of the input current. However, the duty cycle of the switch Sin varies with different outputs and the actual input current supplied by the power source is the average value of the current flowing through the switch Sin because of the large input capacitor Cin. Therefore, the peak value of the input current does not mean the same current supplied by the power source. When the output voltage is low, a large input current range is wasted.
In order to limit the input current, the peak current limit could also be used to directly limit the peak value of the current supplied by the power source. A disadvantage of this method is that the current supplied by the power source cannot follow the input current flowing rapidly through the switch Sin. Therefore, it cannot be used directly to control the input current, which is a critical parameter on a per cycle basis.
FIG. 2 illustrates another prior art circuit for current limiting. A resistor R and a switch S are coupled between the power source and the power device. The resistor R is used to sense the current supplied by the power source. The “on” resistance of the switch S is regulated according to the sensed current so as to limit the current. In this method, the resistor R cannot be integrated in the same IC with the switch S and other control circuits because it is used to sense and must be very accurate. So the complexity and cost of the whole circuit are increased. The switch S will also increase the power loss and may cause thermal issues.